Lignin is a complex high-molecular weight polymer occurring naturally in close association with cellulose in plants and trees. Lignin constitutes, on dry weight basis, approximately 27 to 33 percent of the tree in softwoods and approximately 20 to 24 percent in hardwoods. Lignin is amorphous and has a high molecular weight with a tridimensional structural network. Unlike cellulose, lignin is aromatic in nature.
In the paper-making industry, lignin is separated from the cellulose of the wood product by two principal pulping processes known as the sulfite process and the kraft process. In the sulfite pulping process, lignin is separated from the cellulosic portion of the wood pulp by direct sulfonation, while the kraft process is based on an alkaline degradation mechanism causing cleavage of .beta.-aryl ether linkages in the polymeric lignin which subsequently results in chemical functions of the phenolic type. Kraft lignin is isolated by acid precipitation from the black liquor of the kraft pulping process at a pH below the PKa of the phenolic groups. Purification of the sulfite lignin involves fermentation of residual sugars from the wood to alcohol and the exchange of calcium or magnesium ions to sodium. The kraft process is more widely employed and constitutes about 60% of the worldwide pulping processes employed in the paper-making industry.
Lignin by-products of the black liquor residue of a kraft pulping process employed in the paper-making industry have long been employed as additives in other chemical applications and compositions. The high degree of chemical activity which is characteristic of lignins permits the preparation of many novel and economical organic derivatives. Typical reactions which lignins can undergo are hydrogenation, halogenation, nitration, sulfonation, oxygenation, salt formation, etherification, and esterification. Lignin is a naturally occurring polymer characterized by a series of closely linked benzene rings carrying methoxyl, hydroxyl, and other substitute groups. Lignin by-products variously have been employed as additives in various chemical compositions as a surfactant, extender, dispersant, reinforcement, absorbent, binder, sequestering agent, emulsifier and emulsion stabilizer, and as a stabilizing and protective colloid. Lignosulfonate compounds, particularly sodium salts of lignosulfonates, have been employed as additives and dispersants in textile dyestuffs and printing pigments. Such lignin by-products have been sold for several years under the trademarks Indulin.RTM., Reax.RTM., and Polyfon.RTM. by Westvaco Corporation of North Charleston, S.C.
In the kraft pulping process, lignin is obtained as a by-product from the spent pulping liquor, known as black liquor, where lignocellulosic materials, such as wood, straw, cornstalks, bagasse, and the like are processed to separate the cellulosic pulp from the lignin. In kraft pulping, the wood is subjected to the effects of strong alkali wherein the lignin forms a soluble sodium salt in the alkaline region which is separated from the cellulose and dissolves in the pulping liquor. The lignin is then recovered from the black pulping liquor by reducing the pH of the same.
Reduction in the pH of black liquor containing soluble lignin salts generally may be accomplished by introduction of carbon dioxide which converts the phenolic hydroxyl groups on the lignin molecule, which are in ionized form, into their free phenolic or acidic form. This conversion renders the lignin insoluble in the black liquor and, as a result, it precipitates out. To precipitate the alkali lignin from the black liquor as water-insoluble products, the pH of black liquor initially having a pH around 13 is lowered to a pH of about 10.5 at which point the lignin begins to precipitate. The lignin precipitate can be further purified by reducing the pH level to about pH 2 where the lignin is coagulated and washed with water to remove inorganic salts and other impurities and obtain a lignin product designated a "A" lignin. Such products are sold under the name and trademark INDULIN.RTM. "A" by Westvaco Corporation.
Lignin obtained from the kraft, soda, or other alkaline processes is not recovered as a sulfonated product, but is sulfonated, if desired, by reacting the material with a bisulfite or sulfite compound. Sulfonated lignins are understood to be lignins containing at least an effective amount of sulfonate groups to give water-solubility in moderately acid and higher pH solutions.
One conventional process for sulfonating kraft process lignins involves sulfomethylation of the alkali lignin by reacting the lignin with sodium sulfite and formaldehyde. Such a process is described in Adler, et al. U.S. Pat. No. 2,680,113. Sulfomethylation acts upon the aromatic phenolic nuclei of the lignin mulecule in such a manner that --CH.sub.2 SO.sub.3 H groups are bonded to the aromatic ring. It is also possible to sulfonate the lignin side-chain of the aromatic nucleus by sodium sulfite treatment of the lignin in the absence of formaldehyde. Sulfomethylation of the alkali lignin has generally been carried out at a pH level of 9.0 or higher in order to ensure optimum phenol ionization and solubility of the lignin for sulfomethylation.
More recently, it has been proposed to sulfomethylate kraft process lignins in a two-step operation wherein the ionized phenol component of the lignin is methylolated at an alkaline pH by the addition of an aldehyde, the pH is lowered to an acidic pH to precipitate the methylolated lignin to wash the precipitate to remove undesired inorganic salts, and the lignin is thereafter sulfonated by addition of a sodium or ammonium salt of a sulfur and oxygen-containing compound. Such processes are described in commonly assigned co-pending U.S. patent application Ser. Nos. 06/657,973 and 06/679,901, filed Oct. 5, 1984 and Dec. 10, 1984, and now U.S. Pat. Nos. 4,590,262 and 4,642,336, issued May 20, 1986 and Feb. 10, 1987, respectively. Such sodium and ammonium lignosulfonates find use as additives and dispersants in dye compositions and the like.
In the sulfite pulping process, the lignin in the wood undergoes hydrolysis and in situ sulfonation allowing it to become soluble. A variety of commercial sulfite pulping processes exist today and their names reflect the conditions of pulping applied. Among the sulfite processes are the magnesium base pulping process, the calcium base process, the ammonia base process, the soda, the neutral, the bisulfite, and acid sulfite processes. Sulfonation of lignin during the sulfite pulping process takes place in the side chain involving either the terminal alcohol group or the sulfonic acids attach themselves on the carbon of the side chain of the aromatic nucleus. Sulfite lignins are sulfonated during the pulping process in which sodium or calcium bisulfite is used as the cooking chemical. They become water-soluble and thus can be easily washed from the cellulosic pulp.
It has been proposed to employ sodium salts of lignosulfonates as a sequestrant in liquid pesticide formulations; however, their use has been restricted to very low loading levels since higher levels of the sodium lignin salt causes an ion-exchange reaction in amine-containing pesticides with resultant sludging and precipitation in the pesticide liquid composition. Amine salts of ethylene diamine tetraacetic acid (EDTA) and citric acid also have been traditionally used as sequestrants, but their manufacture and use is quite costly.
U.S. Pat. No. 3,784,493 discloses the manufacture of amine lignosulfonates from spent sulfite liquor of the sulfite pulping process which are reacted with formaldehyde to produce products useful as setting control agents in cement, as hydration shale inhibitors in oil well drilling muds, and as fillers in the formation of resins.